This Laser Gun Ammunition Measurement Equipment (Laser GAME) relates to an apparatus for measuring the velocity of a projectile, for example, a bullet shot from a gun, while it is in the gun barrel and/or while it is traveling outside of the gun barrel. This invention utilizes the phenomena of Doppler frequency shifting of the laser light reflected from the projectile as a measure of the velocity of the projectile. Likewise, by providing an appropriate means for integrating and differentiating the velocity data so generated, the instantaneous position and the instantaneous acceleration of the projectile can also be determined.
Several attempts have been made to provide an apparatus which can accurately measure the velocity of a projectile, both while it is within the gun barrel, and after it has cleared the muzzle of the weapon.
Ballistic performance is measured by both passive optical and active microwave devices. Such passive optical devices are known as sky screen devices and light boxes. These optical devices are used to provide a measure of average velocity between specific survey points by measuring the time required for the projectile to traverse a predetermined distance. The microwave devices include radar equipment with the associate radar transmitter, radar receiver and radar signal processor. A recent development is an x-band radar which detects the ballistics velocities with conventional Continuous Wave (CW) radar, operating at 10 GHz. The Doppler radar provides instantaneous velocities with some inhereal Continuous Wave (CW) radar, operating at 10 GHz. The Doppler radar provides instantaneous velocities with some inherent limitations. Both the optical and microwave devices are capable of evaluating the ballistics after the projectiles leave the gun muzzle. This will be referred to as "external ballistics", meaning external to the gun barrel. The microwave devices are also capable of measuring ballistics internal to the gun barrel, i.e. internal ballistics. Several factors limit the performance of these systems for both external and internal ballistics measurements.
Since conventional passive optical techniques for ballistics velocity measurements are limited to external evaluation of average velocity between predetermined (survey) points, the necessity of locating sky screens (outdoor range) or light boxes (indoor range) at various survey points limits the number of measurements which may be taken to the number of pairs of test stations (screens or boxes). The velocity measured is actually an average velocity defined by the distance between survey points covered over some measured time interval. The resolution of the velocity measurement increases with an increasing number of test stations; however, increasing the number of test stations reduces the accuracy of the velocity measurement because of limitations in the ability to accurately measure the time intervals between ever decreasing distances. The result is that the measurement resolution varies inversely with accuracy. Furthermore, any ballistic velocity measurement conducted under the sky screen/light box approach, is limited to the evaluation of average velocity valid only between the test stations. Half of the time the actual projectile velocity is greater than the average velocity, and half the time the projectile velocity is less than the average. The utility of the average velocity measurement must be determined by the user. It may be valid as a measure of the repeatability of a weapon or ballistic; however, it may not be valid for ballistic performance at a point which is outside of the interval of measurement, nor for evaluating rapidly evolving ballistics, within the measurement interval.
Recently, the use of microwave radar for ballistic measurements has been used to attempt to overcome the shortcomings of average velocity measurements made with the sky screens/light boxes of the passive optical type. The microwave radar uses X-band transmitters (nominally at 10 GHz) to illuminate ballistic targets, from a near tail-on aspect, and then the radar receiver collects the frequency shifted, reflected energy. This frequency shifted, reflected power is known as the "Doppler return". The Doppler return is mixed with a local oscillator and detected in the radar receiver. The results of mixing the return power with that of the local oscillator give rise to additional frequencies, commonly referred to as the sum and difference frequencies. However, only the difference frequency is of interest, since it is within the electronic bandwidth of the radar receiver's detector. This Doppler frequency shift gives a direct measure of the projectile velocity and is evaluated by means of a detector and a signal processor.
External ballistics measurements utilizing the microwave radar permits measurement of the projectile velocity through continuous wave (CW) Doppler radar techniques. However, the radar is inadequate for measuring velocities near the muzzle due to the opacity of the fire ball and persistence of the resulting plasma. The X-band (10 GHz) measurement is also suited only for larger caliber ballistics, since the target radar cross section becomes a highly random parameter as the projectile dimension approaches the wavelength of the incident electromagnetic energy beam. Microwave radiation has a wavelength in the range of 10 mm to 3 meters. At 10 GHz the wavelength is approximately 30 mm. For target dimensions near or below 30 mm (such as the projectile diameter), the X-band radar is simply not well suited for making projectile velocity measurements.
In addition, the above microwave methods are not totally compatible with measuring ballistics of a projectile while it is in the gun barrel. When the barrel diameter is approximately equal to the wavelength, the barrel acts as a waveguide, and as the projectile moves through the barrel, it tunes the cavity, resulting in a series of standing wave patterns. A detector then detects the serial maximum and minimum patterns as the projectile accelerates down the barrel. This measurement technique resembles the stationary light boxes of the passive optical art, since the standing wave patterns give rise to a discrete number of measurements as the projectile moves down the barrel. The waveguide nature of these measurements are dependent on a barrel diameter to wavelength relationship and a repeatable transmitter and barrel geometry.
Several U.S. patents describe the measurement of projectile velocity either within or outside of a gun barrel.
Smith, in U.S. Pat. No. 2,691,761, describes a system for measuring internal gun ballistics (within the gun barrel) using a microwave transmitter. The system of Smith requires a substantial amount of modification to the gun barrel. Furthermore, the system of Smith requires tuning of the microwave pattern and cleaning of the gun barrel after each round is fired. These features of the system make its use very inconvenient.
Schultz et al., U.S. Pat. No. 2,735,981, describes another microwave transmitter system used to measure the projectile velocity while in the gun barrel. Again, tuning of the standing wave is required. Also, Schultz employs an expendable microwave element which must be replaced between shots. This adds considerably to the cost and complexity of using this apparatus.
Elgaard, U.S. Pat. No. 3,918,061, shows a system for measuring projectile velocity external to the gun barrel. This system uses a microwave radar source and detects the Doppler return signal. However, microwave wavelengths are strongly attenuated by the plasma and also by the by-products of the burning propellant (Rayleigh scattering). The persistence and density of both the plasma and the neutral particles which emanate from the barrel, adversely effect the transmission of the radar. Accordingly, measurements of the projectile velocity very near the muzzle are not possible with this system. It is also not possible to make internal ballistic measurements with the system of Elgaard.
Toulios et al., in U.S. Pat. Nos. 4,283,989 and 4,457,206 show a microwave system for measuring projectile velocities both internal and external to the barrel. The microwave sources require tuning of the standing waves, as noted above. Also, the system of Toulios requires non-trivial modification of the gun barrel. Additionally, as noted above, microwave wavelengths are significantly limited in their ability to propagate through the plasma and by-products of the burning propellant. Furthermore, microwave radar emits a broad angular beam, and this beam may be unintentionally intercepted or detected by another party in a combat situation. This broad beam also increases the probability of cross-talk between closely spaced units.
Schmidt, U.S. Pat. No. 4,486,710, shows a coil attachment to the gun barrel which makes a single measurement of the velocity of the projectile within the gun barrel. No external measurements are possible. Again, significant barrel modifications are required.
All of the above-mentioned patent documents, and any patent document mentioned hereafter are entirely incorporated herein by reference.